This application relates generally to rechargeable medical devices, and this application relates more specifically to electrically powered implantable medical devices (IMDs) that include rechargeable batteries that are charged through the skin using inductive charging techniques.
The electrical circuits of IMDs are used to monitor the patient, disperse medications, and also to apply therapeutic electrical stimulation to tissue. Electrically driven implantable devices are used, for example, as neuro-stimulators including pain suppression, hearing aids (e.g. cochlear devices), cardiac pacemakers, and defibrillators. However, IMDs may also be used for drug infusion and dispensing systems, nerve and bone growth stimulators, digestive track stimulators, artificial vision apparatus, artificial organs including artificial hearts, bladder stimulators, and for the purposes of implanted sensors that monitor but do not actively stimulate tissue. Additionally, IMDs have been used as combinations of the above listed devices, such as a combined cardiac pacemaker and cardiac defibrillator. Thus, the electrical circuitry can provide therapeutic electrical stimulation of tissue, monitor the patient, and dispense medication.
These IMDs often have power requirements in excess of what can be provided by conventional batteries (without cumbersome replacement processes), thus rechargeable batteries are often provided in the implanted device. This prevents the need for electrical wiring that protrudes from a patient's skin, which presents an infection hazard. Therefore, implant rechargeable batteries are often charged via an inductive signal provided by an external charger to a charge receiving coil of the implant. This enables recharging of the battery through the patient's skin without physical contact between the implant and the external charging unit.
However, the traditional inductive recharging process for IMDs requires that a user sit relatively still during the recharging period to maintain alignment between the IMD and an external charging unit which provides the inductive signal that is converted into electricity for charging the IMD internal battery. This can interfere with the user's lifestyle.
Additionally, newer implants that include neuro-stimulators, often consume relatively large amounts of power in comparison to older pacemakers which provide low power electrical stimulation signals. These newer implants require more frequent charging, with some requiring a recharging interval of two weeks or less, and thus patients expend a considerable amount of time charging their IMDs.
In the past, charging times for the rechargeable batteries for IMDs were limited by a number of factors including battery chemistries. However, with the introduction of newer lithium ion batteries and other batteries, battery chemistry is no longer the primary limitation to recharging of IMDs. Instead, temperature limitations inside the human body that result from the recharging process of the implant are becoming an increasingly problematic limitation that prevents faster battery charging.
During charging, some of the inductively transferred energy provided to the IMD is converted into heat as opposed to being converted into electricity for recharging. Eddy currents form in the housing of the IMD during charging and these currents are converted into undesirable heat. Also, some of the inductive signal in the recharging circuitry within the IMD is also converted into heat. As a result, this waste heat will raise the temperature of the IMDs, potentially to undesirable levels if not properly monitored. Conventional approaches for controlling temperature during inductive charging typically included completely shutting down charging if an excessive temperature level is reached, or charging the implant's battery at relatively low charging rates to avoid exceeding temperature limitations. However, shutting down the charging process is inefficient and can increase charging times and reduce charging efficiencies. Charging at lower charging rates also increases the charging times for IMDs.
Desired is a way to overcome one or more of the shortcomings of the prior art described above.